I don't have problems with drag'n'dropping under XP. If you're going to install Zen Media Explorer you'll need to enable one option to show Zen as Portable Media Device, but it probably also applies to Vista. It won't work as UMS it is always MTP, but Vista and XP with at least SP1 treat this protocol similar to additional drive.

yea, i agree. XP works fine with drag and drop. like valdr said, you need to go to the zen media explorer settings and allow it to show the portable media for drag and dropping. actually, you can drag and drop with the zen explorer too.

If you're planning to get files from friends using XP there shouldn't be any problem, you'll probably see the Zen as PMD right after plugging it. You don't need to install any Creative software. Pictures and audio should be ready to play after transferring, but videos usually need transcoding (see Video subforum for details).
I can't confirm that but it seems that Zen Media Explorer works even better with XP especially if you'd like to drag'n'drop album art with it.